If the planer's bed is adjusted parallel to the cutterhead, then the cutterhead becomes a datum reference surface too, so setting the knives parallel to the cutterhead will make them parallel to the bed. It's also easier to use the cutterhead as a reference surface because you can work from the top rather than indicate from underneath, rotate the cutterhead up, adjust and re-tighten the knife, kneel down, rotate the knife down, and check the new setting. Been there years ago. Didn't like it much. {Reference:http://owwm.org/viewtopic.php?t=197731|http://owwm.org/viewtopic.php?t=197731}